Hydromechanical properties of shales are complex due to the involved ma-terial structure, with the solid matrix being mainly formed by swelling clays and porosity dominated by nanometer scale tortuous voids with large aspect ratios. Intrinsic permeability of restructured Opalinus Clay (Swiss shale) brought to shallow geological storage conditions was measured with in situ brine. Under constant temperature, vertical stress, and downstream fluid pressure, steady-state flow experiments show a significant trend of permea-bility decrease with increasing differential (upstream minus downstream) fluid pressure, thus contradicting the conventional Darcy’s description. To interpret these experimental measurements, brine permeability is derived using a one-step self-consistent homogenization scheme based on the knowledge of material’s pore structure. While mechanical and thermal effects cannot ex-plain the permeability decrease, the trend is reproduced with the correct order of magnitude by considering a chemical effect: a pore size reduction in the sample due to water adsorption at mineral surface.